You cannot fully prevent decompilation of a GM8 game. The runner is inherently insecure. However, you can make it annoying.
Legacy (circa 2012) Difficulty: Easy (Drag-and-drop)
A classic open-source tool; many modern decompilers are based on its original logic. GM 5.3A – 7.0 gamemaker 8 decompiler
Originally designed for the MMO Last Moon , this tool evolved into a robust GM8.1 decompiler and editor. Unlike others, it allows you to repack the game after modifications without needing the original project file. It is one of the few actively maintained tools for the GM8 ecosystem, mostly kept alive by furry gaming communities and classic MMO private servers.
Here is the harsh reality:
The is a double-edged sword. It is a powerful educational tool that built a generation of indie developers who learned by reading the masters' code. It is a preservation tool that saves forgotten games from bit rot. But it is also a weapon for plagiarism and asset theft.
If you are a developer still distributing a GameMaker 8 game (perhaps on itch.io or Steam), you need to know how to frustrate decompilers. You cannot fully prevent decompilation of a GM8 game
The GameMaker 8 decompiler is a powerful tool that can recover GML code and assets from compiled GameMaker projects. While it has limitations, its capabilities raise concerns about intellectual property protection and code security. By understanding the decompiler's capabilities and taking proactive measures to protect their work, game developers can minimize the risks associated with the decompiler and ensure the security of their projects.